New Research: Tracking the Birthplaces and "Lost Years" of Green Turtles

New research led by scientists at the American Museum of Natural History and the City University of New York provides insight to the population distribution and “lost years” of Central Pacific green turtles, the span of time when the turtles hatch, enter the water, and emerge on a feeding ground often hundreds of miles away.

A green turtle (Chelonia mydas) in the Palmyra Atoll National Wildlife Refuge

The study indicates that instead of simply drifting with ocean currents until they reach a landmass, young sea turtles may actively swim to reach specific feeding grounds. Published in the Journal of the Royal Society Interface, the work provides information necessary for conservation and management of this vulnerable species.

“Green turtles are highly migratory,” said lead author Eugenia Naro-Maciel, a researcher affiliated with the Museum’s Center for Biodiversity and Conservation (CBC) and an assistant professor at the College of Staten Island, City University of New York. “After hatching from eggs on nesting beaches, young turtles disperse into the ocean. Years later, as juveniles, they appear on coastal feeding grounds. We want to learn more about the connections between nesting and feeding areas because it’s vital to know the full range of an organism in order to manage it.”

The researchers focused their study on the green turtles (Chelonia mydas) at the feeding grounds in the remote Palmyra Atoll National Wildlife Refuge, located about halfway between Hawaii and American Samoa.

“Sea turtles at Palmyra forage in a unique environment that's removed from pervasive human influence,” said Eleanor Sterling, director of the CBC and an author on the study. “Our studies are highlighting the importance of Palmyra as a green turtle feeding ground. The current research provides a clearer understanding of where the turtles foraging on Palmyra come from and go to. Our results may help pinpoint the geographical regions needing additional conservation management.”

In order to learn more about the migratory paths green turtles take to get to Palmyra, the researchers collected tissue samples from about 350 turtles in the wildlife refuge over the course of five years. DNA analysis of these tissues indicated that almost all of the turtles (about 97 percent) were born in the west central and south central Pacific, where Micronesia and American Samoa are respectively located. Then, an ocean current model was used to distribute thousands of virtual particles in the water—imitating newly hatched turtles a little larger than ping pong balls (5 centimeters)—and follow them back in time three years to their likely birthplace.

A diver with a green turtle (Chelonia mydas) in the Palmyra Atoll National Wildlife Refuge

The ocean current model gave a very different answer for the turtles’ origins, predicting that about half were born in the west central and south central Pacific and half were born in the east Pacific (Galapagos and Mexico).

“Based on our genetic data, we know that there’s no way half of the turtles came from the Galapagos and Mexico, it’s more like 3 percent tops,” Naro-Maciel said. “We think the genetics are correct, so what’s missing from the model? The most likely explanation is that even though these turtles are very small, they are exhibiting slight swimming behavior that’s having a major impact on their course.”

This study supports a growing consensus that marine animals do not randomly search out food, but instead follow somewhat-fixed migratory routes.

“Ocean circulation models show the pathways that lead to Palmyra, but it appears that turtles of some populations choose to get on those roads and others do not,” said Nathan Putman, a researcher at Oregon State University who carried out the modeling component of the study. “It certainly implies that turtles have some control over where in the ocean they travel.”

More research is needed to determine how turtles navigate this path, whether it’s through cues from magnetic fields or some other mechanism. But knowing more about where green turtle migratory pathways are distributed in the Pacific and the tools that are able to expose them is a step toward comprehensive conservation of the species.